The present disclosure relates to systems which monitor flow of fluids, namely gases, in a duct. More specifically, the present disclosure relates to duct flow primary elements, transmitters and systems of the type which provide duct flow measurements.
In industrial settings, control systems are used to monitor and control inventories of industrial and chemical processes, and the like. Typically, the control system that performs these functions uses field devices distributed at key locations in the industrial process and coupled to control circuitry in the control room by a process control loop. The term “field device” refers to any device that performs a function in a distributed control or process monitoring system, including all devices used in the measurement, control and monitoring of industrial processes.
Some field devices include a transducer which couples to the process fluid. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. Typically, a transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, actuators, solenoids, indicator lights, and others.
Field devices, such as process variable sensors used in industrial processes can be installed in the field on pipelines, tanks and other industrial process equipment like ducts. Such devices sense process variables such as process fluid flow, process fluid temperature, process fluid pressure, process fluid conductivity, process fluid pH and other process variables. Other types of industrial process field devices include valves, actuators, field controllers, data displays and communication equipment such as industrial field network bridges.
Duct flow measurements are commonly needed in industrial process or other applications. In duct flow measurement applications, reliable and repeatable measurements are typically required, while total measurement uncertainty is often a secondary consideration in this type of application. Commonly used duct flow measurement technologies include thermal mass flowmeters and pitot arrays. Thermal mass can be an accurate method of measuring flow in ducts. Unfortunately, the technology is susceptible to inaccuracy in applications with high humidity and can be difficult to verify. Pitot arrays are based on differential pressure (DP) principles and are therefore reliable, repeatable, and easily verified. However, they are expensive and need to be fabricated based on customer provided dimensions. This fact results in pitot array solutions with long lead times and requirements for customer supplied information inputs to the manufacturing process.
The averaging pitot tube (APT) is a type of primary element commonly used for flow measurement because of its ability to be inserted into and retracted from a duct or pipeline, its low pressure loss, and reliable performance. The APT primary element generates and averages pressures from multiple locations across a duct or pipeline through which a process fluid or gas is traveling. This average pressure is then used in conjunction with flow theory and experimentally determined quantities to provide a flow measurement for the fluid or gas. One type of APT primary element is the Annubar® APT available from Emerson Process Management.
One known solution for duct measurement is to use a relatively high cost APT primary element, such as the Annubar® APT, with a duct flange to couple the APT primary element to the duct. Such a solution is composed of relatively high cost parts designed for more severe applications such as pipeline fluid flow measurement. Using this type of APT primary element for making duct flow measurements, where total measurement uncertainty is not as critical as in other types of flow measurement applications, results in a relatively costly solution. Further, the APT primary element distribution model used by manufacturers typically requires communication of process conditions and duct dimensions through an ordering chain, pre-order sizing, and custom manufacturing processes. These limitations further increase costs, limit service level, and mitigate profitability to manufacturers.